1. Field of the Invention
The present invention relates generally to compositions for use in investment casting. More particularly, the present invention is an ashless, foamable investment casting composition which yields a smooth interior mold surface.
2. Discussion of Background
Investment casting is a well known industrial procedure used in the manufacture of various metal parts, usually ones having intricate shape such as aerospace or automotive parts and military applications. Investment casting is also used for the manufacture of plumbing fixtures, air conditioner compressor parts and various consumer goods. In this procedure, a series of replicas, normally made of a wax-based composition or a sand and binder composition, are made in the shape of the part to be created. This replication is accomplished by heating and subsequently pouring the wax-based or sand and binder composition into a metal mold and allowing the replicas to cool. Once the replicas are fully cooled, they are removed from the metal mold. These replicas are then dipped into a slurry of refractory mold material, or otherwise coated, to form a refractory shell about the replicas. The constituents of the slurry are of a small particle size so that a fine interface surface is established. After the refractory shell is dried, it is dipped in a second refractory slurry, composed of larger sized particles, to give the shell adequate thickness and handling strength. Once the ceramic mold is completely dried, it is subjected to a thermal schedule. During this heating process, the replicas are "burned out" of the mold, leaving a cavity in the shape of the part to be manufactured. After the replicas are removed, molten metal is poured into the cavities of the ceramic mold to form the parts in the desired shape. After the metal cools and hardens the ceramic mold is removed by cracking or breaking.
One problem encountered in the art of investment casting is improperly shaped final parts. When the heated replica composition is introduced to the metal mold, and subsequently allowed to cool, thermal contraction takes place and the hardening replica shrinks from the interior sides of the mold. This shrinking causes the replica to loss its geometrical integrity and often causes the replica to lose its shape. As a result, many ceramic molds fail to maintain proper dimensions, which in turn results in inadequately shaped parts that fail to meet specifications.
Another problem currently faced by the industry is irregularities in the surface surrounding the cavity of the mold. These surface irregularities are formed during the sintering process and are the result of ashes (generated by burning replicas) that lay on, or are imbedded into, the ceramic mold. These ashes leave indentations, which in turn result in surface defects in the final metal part. These defects often prevent the metal part from meeting required specifications and tolerances, which in turn increases production costs.
Still another problem encountered by the industry is cracking of the ceramic mold during the "burning out" phase. When the replica, positioned within the ceramic mold, is subjected to heat, thermal expansion of the replica causes the exertion of a pressure about the interior of the ceramic mold. This pressure, if not alleviated, often causes the ceramic mold to crack. This premature cracking causes an increase in both production time and costs.
Therefore, there exists a need for an ashless, foamable investment casting composition.